Over the past two decades the development of novel drug delivery systems has had a tremendous impact on medicine, allowing for the improvement of many existing therapeutics as well as enabling the use of entirely new therapies. Along with these improvements there has been a move to miniaturize drug delivery devices from the macroscale (>1 mm) to the microscale (100-0.01 μm) or nanoscale (100-1 nm). One goal of developing these devices is to provide for the in situ delivery of therapeutic compounds. Therefore, a main area of investigation concerns the development of integrated systems that combine the nanotechnology with therapeutic molecules, for example, drugs, or biopolymers.
However, these new technologies must overcome several crucial and significant obstacles. For example, new approaches are required that allow for the delivery of insoluble, or unstable compounds. In addition, these approaches must address situations in which the therapeutic molecule is rare, or difficult to purify. Furthermore, these approaches must allow for localized delivery of potent compounds, and provide for improved compliance by reducing the chances of missing or erring in a dose. Lastly, these devices cannot be so small that they are unable to deliver an adequate dose.
In addition, to date a number of peptides and proteins have been explored for pharmaceutical applications by virtue of their high biological activity and specificity. Despite potential advantages offered by these drugs, their application may suffer because of the high molecular weight, hydrophilicity and low stability, which are reflected in poor biopharmaceutical properties. In particular, peptides and proteins undergo rapid clearance from the body, which takes place by a combination of events including proteolysis, renal ultrafiltration, and liver clearance.
The development of functionalized polymers that can elicit specific biological responses, as well as the development of methods to fabricate these biologically functionalized polymers, is a viable approach for such therapeutic applications, in which delivery of molecules such as those above is desirable. For example, the generation of fibrous matrices with biological properties and fiber diameters commensurate with those of the natural extracellular matrix (ECM) may permit the development of novel materials for use not only in drug delivery, but also in wound healing or tissue engineering.
Synthetic polymers, which can be designed to mimic some functions of biopolymers, have been actively developed as drug delivery matrices due to their industrial and scientific value. They represent a primary polymeric vehicle for the delivery of drugs and biomolecules, owing to their simple synthesis, ease of processing, and range of physical properties. Polymers such as proteins, polysaccharides, and nucleic acids, which are present as basic components in living organic systems, have also found some use in drug delivery and biomaterials applications, particularly for biopolymers such as collagen and alginate, which have desired mechanical and biological properties. There remains a significant need, however, for the facile production of polymeric materials in which mechanical and biological properties can be easily modified in a modular fashion, without relying solely on the naturally occurring mechanical properties of the biological polymers or the lack of biological activity of synthetic polymers.
One approach to the use of polymers in drug delivery applications involves the response of a polymer system to certain stimuli, which is a common process for biopolymers in living organisms. Stimuli responsive polymers can provide a variety of applications for the biomedical fields. The interest in these polymers has increased dramatically due to their promising potential. Among others, temperature and pH responsive mechanisms have been investigated as they are relatively convenient and effective stimuli in a variety of applications. Such stimuli-responsive polymers have been utilized in various forms, including cross-linked (permanently) hydrogels, reversible hydrogels, micelles, modified interfaces, and conjugated solutions, among others.
There has also been developing interest by the scientific community in applying methods from engineering and the life sciences to create artificial constructs directed to tissue regeneration, and thus, providing a minimally invasive and less painful way to treat patients. The basic protocol for the tissue engineering approach includes isolating specific cells by obtaining them through a biopsy from a patient, growing them on a three-dimensional scaffold under controlled culture conditions, delivering the construct to the desired location in the patient's body and directing new tissue formation into the scaffold that will be degraded over time. Successful regeneration of damaged organs or tissues based on tissue engineering requires several critical elements, not the least of which includes biomaterial scaffolds that serve as a mechanical support for cell growth.
Materials are important in most tissue engineering strategies as they can serve as a substrate on which cell populations can attach and migrate, be implanted with a combination of specific cell types as a cell delivery vehicle, and be utilized as a drug carrier to activate specific cellular function in the localized region.
The modification of biomaterials with bioactive molecules has been found useful in designing biomimetic scaffolds that can provide biological cues to elicit specific cellular responses and direct new tissue formation. The surface and bulk modification of materials with peptide sequences or with bioactive proteins or other biomolecules, for example, can allow for the modulation of cellular functions such as adhesion, proliferation and migration through alterations of peptide concentration or its spatial distribution. A variety of cell binding peptides have been introduced into three dimensional networks through physical, chemical, photochemical, and ionic crosslinking. The molecules have been attached to a variety of polymeric substrates for varying applications, but a need still remains for methods for simple modification of materials with multiple bioactive molecules in a manner that can control their release.
Electrospinning is an attractive approach to polymer processing. The control of fiber diameter, porosity, and fiber surface morphology makes electrospun fibers useful in a range of applications including filtration, electronic, and biomedical applications. Electrospun fibers are ideal for use in biomedical applications such as tissue engineering and drug delivery due to the three-dimensional nanometer scale matrix that can be quickly produced using small quantities of starting material (<50 mg) on the laboratory scale. Larger matrices can also easily be made via the scaling up of materials that prove to have useful materials and biological properties on the laboratory scale. Collagen, fibrinogen, chitosan, poly(lactic acid), poly(L-lactide-co-β-caprolactone), and poly(D,L-lactide-co-glycolide) are just a few of the polymers being investigated for use in electrospun drug delivery and tissue engineering constructs due to their biocompatibility. Electrospun fibers mimic the size scale of fibrous proteins found in the extracellular matrix (50-150 nm) and the three-dimensional nature of the matrix allows for cells to infiltrate the matrix and proliferate. A variety of additional polymers can also be electrospun into matrices for drug delivery, tissue engineering, and other applications. For example, see Z. M. Huang, Y. Z. Zhang, M. K. Kotaki and S. Ramakrishna, Composites Sci. and Tech. 2003, 63, 2223-2253, (US Patent publication No. US20030137069).
Although there have been reports of the use of electrospun fibers as scaffolds for select drug delivery and biomaterials applications, the incorporation of bioactive molecules has not been widely exploited. In one example, tetracycline was included in a solution of poly(lactic acid) (PLLA) and poly(ethylene-co-vinyl acetate) and electrospun in a fiber membrane to study the release profile of the fiber-encapsulated drug. PLLA has also been studied as a matrix to incorporate the drugs rifampin (to treat tuberculosis) and paclitaxel (anti-cancer drug). Polymers loaded with both of these therapeutic molecules could be fabricated into nanometer to micron diameter electrospun PLLA fibers and used for delivery of the drug.
While the development of functionalized polymers that can elicit specific biological responses is of great interest in the biomedical community, as well as the development of methods to fabricate these biologically functionalized polymers, there is still a need for more facile methods to create biologically active functionalized matrices that permit immobilization and long-term delivery of biomolecules and other small functioning molecules, moieties, and/or particles.
For example, although there have been studies demonstrating the utility of functionalizing electrospun fibers, the functionalization of electrospun fibers to confer specific biological activity would be particularly advantageous for many biomaterial applications, as such modification would permit the fabrication of biomaterials that are structurally relevant and that possess properties designed to treat or address specific problems. In addition, such incorporation could serve to protect bioactive molecules, release them in relevant timeframe, and incorporate multiple such molecules. In particular, the development of simple methods to produce multifunctional electrospun fibers would advance the development of electrospun fibers for biomedical applications.
Of such bioactive molecules, glycosaminoglycans have been demonstrated to mediate a wide range of biological activities such as cell adhesion, cell mobility, cell proliferation and tissue morphogenesis via binding to various cell regulatory proteins such as the chemokines, growth factors, enzymes, enzyme inhibitors, and extracellular matrix proteins. The well studied glycosaminoglycan, heparin, is a linear, unbranched, highly sulfated polysaccharide chain, and it is well accepted that the electrostatic interactions between the sulfates of the glycosaminoglycan and basic residues of a protein play an important role in binding. In particular, the spatial orientation of the basic residues is a major determinant of heparin-binding ability, and variations in the pattern of the sulfation of the heterogeneous heparin therefore permit binding between heparin and a wide range of binding partners. Accordingly, heparin has been incorporated into covalent hydrogel delivery systems because of this ability to bind a diverse set of proteins. In addition, we and others have demonstrated that the interactions between heparin and specific HBPs can also mediate the assembly of noncovalently associated hydrogel networks (see, for example, N. Yamaguchi, K. L. Kiick, Polysaccharide-poly (ethylene glycol) star copolymers as scaffolds for the production of bioactive hydrogels, Biomacromolecules 6 (4) (2005) 1921-1930; N. Yamaguchi et al., Rheological characterization of polysaccharide-poly(ethylene glycol) star copolymer hydrogels, Biomacromolecules 6 (4) (2005) 1931-1940; B. L. Seal, A. Panitch, Physical polymer matrices based on affinity interactions between peptides and polysaccharides, Biomacromolecules 4 (6) (2003) 1572-1582).
Because hydrogels can mimic the high water content and mechanical properties of natural tissues, they are prime candidates as carriers of bioactive agents, in bioadhesive systems, or as biorecognizable materials. Given that poly (ethylene glycol), or PEG, is highly hydrophilic and generally nonadhesive to proteins or cells, it has found widespread use as a drug carrier, and many PEG hydrogels have been produced from aqueous solutions containing linear or branched PEG macromolecules via chemical crosslinking. In addition to hydrogels formed via radical crosslinking reactions, PEG hydrogels have been formed, for example, via Michael-type addition reactions upon mixing with thiol-bearing compounds or via the reaction between amino-terminated poly (ethylene glycol) and the herbal iridoid glycoside genipin. In some cases, cell adhesive peptide domains or biodegradable sequences have been introduced into PEG hydrogels to endow them with more biological signaling functions, including the capacity for growth factor delivery. Accordingly, there have been a variety of covalently crosslinked hydrogel systems developed to deliver growth factors via mechanisms such as diffusion and chemical or enzymatic reaction.
Noncovalent interactions provide an alternative method for crosslinking, as introduced above, removing the need for toxic chemical crosslinking agents in gel preparation. In addition, noncovalent crosslinking strategies may offer advantages in maintaining protein integrity and bioactivity until delivery. Polymer hydrogels have been formed via specific recognition events such as reversible antibody-antigen interactions (see T. Miyata et al., A reversibly antigen-responsive hydrogel, Nature 399 (6738) (1999) 766-769) and coiled-coil interactions (see for example, C. Y. Xu et al., Reversible hydrogels from self-assembling genetically engineered protein block copolymers, Biomacromolecules 6 (3) (2005) 1739-1749; C. Wang et al., Hybrid hydrogels assembled from synthetic polymers and coiled-coil protein domains, Nature 397 (6718) (1999) 417-420; W. A. Petka, et al., Reversible hydrogels from self-assembling artificial proteins, Science 281 (5375) (1998) 389-392). There has been little attention given, however, to the interaction between peptides (particularly coiled-coils) and polysaccharides in hydrogel assembly. Further, the fabrication of biologically active fibers through such functionalization strategies is a fairly new and unexplored area in the field of electrospinning, and electrospun fibers have not been investigated for their ability to incorporate growth factor binding glycosaminoglycans, such as heparin, in a manner designed to control release.